This invention relates to roof bolts or rockbolts for supporting the roofs of underground excavations, such as tunnels or mines.
Rockbolts are long bolts in the roof of an underground excavation for support. The upper end of the rockbolt is secured in a vertical bore in the roof, while the lower end of the rockbolt engages a faceplate which engages the roof. Rockbolts are not usually used alone, but rather are arranged in a grid-like network in the area to be supported, normally spaced apart in two to four foot intervals. Thus, in a twenty by thirty foot area it would not be uncommon to have sixty or more rockbolts.
Broadly, there are two types of rockbolts: untensioned and tensioned. Untensioned rockbolts engage the rock along substantially the length of the bolt and accept load as the roof deforms, supporting the roof by engaging the overlying rock. Tensioned rockbolts engage the rock at the upper end and at the lower end by a faceplate, and are put in tension which compresses the surrounding rock. In the grid-like pattern in which the bolts are used, this creates ribs of compressed rock across the roof span, which actually act like beams.
Rockbolts have been used for many years, during which time many types have been developed. A good description of prior rockbolts can be found in Underground Excavations in Rock by E. Hoek and E. T. Brown, incorporated herein by reference. The first development in untensioned rockbolts was a tight-fitting dried wooden dowel which after insertion would absorb moisture from the surrounding rock and swell to frictionally engage the rock. Wood, however, was weak and it could not be tensioned, so this was an unsatisfactory solution. Various types of metal bolts with expanding anchors have been developed. One such bolt is the Worly bolt, which has a plurality of ramps machined or cast into the bolt, and an anchor member which is driven up the ramps, expanding the bolt. This type of bolt is expensive to manufacture and difficult to install because the bore size was critical to proper engagement. They could not be tensioned and they were subject to corrosion.
The most recent development in untensioned rockbolts is an inflatable rockbolt. One such bolt is commercially known as the Swellex bolt. A collapsed tube is inserted in the bore and high pressure applied to inflate or expand the tube into secure engagement with the bore. Expansion of the tube puts radial stress on the rock which causes it to crack. This bolt can corrode, it is expensive, and it cannot be tensioned. The strength of the bolt is also limited because the walls must be thin enough to flex.
Grout has been used to secure metal rods or wooden dowels in the bores. Grout provides good engagement between the rod and rock and also protects the rod from corrosion. However, large quantities of grout are needed to fill the space between the rod and the rock otherwise larger, more expensive rods would be needed. Furthermore, the bore is usually drilled longer than the rockbolt, and the insertion of the rod pushes a lot of grout up into the dead space above the rockbolt. Grout is very messy and difficult to handle. It generally must be made at the site of use and near the time of use, so extra personnel are needed just to handle the grout. Special equipment is also required to prepare and pump the grout. Even so, large amounts of grout are wasted. Further, the grouted rods generally cannot be tensioned.
One type of grouted rockbolt, often called a perfobolt, consists of two perforated half tubes filled with grout, wired together and inserted in the bore. A rod is driven into the tube, extruding the grout through the perforations. Such a bolt was shown in Flygare, et al., U.S. Pat. No. 2,849,866, which is incorporated by reference herein. This type of bolt was time-consuming to install, and required special equipment, and still could not be tensioned.
More recently, cartridges of hardenable resin and catalyst have been inserted into the bore and punctured and mixed by inserting and turning the rod. This achieves a very secure engagement, but the resins are very expensive and large amounts were used, especially in the larger bores. The problem of pushing the resin into the dead space above the rod increases the amount of resin used. There are difficulties in inserting the cartridges and the rod into the bore and special equipment was often needed. It is difficult to adequately mix the resin so that it sets properly. Adequate mixing is dependant upon the size of the annulus between the bore and the rod, so bore size and rod size are critical. Adequate mixing also requires that the rod be rotated for the prescribed time. This is time consuming, and quality can vary from bolt to bolt, ultimately depending upon the particular installer.
An example of a resined rockbolt is shown in Lang, U.S. Pat. No. 4,098,166, which is incorporated by reference herein. The Lang device has a specially designed rod and faceplate to facilitate the installation of the rod and the mixing of the resin.
Tensioned rockbolts are generally preferred over the untensioned types and the first development in this area was a metal rod having a slotted upper end with a wedge disposed in the slot. The end was secured in the bore by pushing the assembly against the end of the bore to drive the wedge in and expand the rod end. This provided only a small contact area so the rod could slip, and bore length and diameter were critical to proper installation. Corrosion of the rod was also a problem. A mechanical anchor or expansion shell on the end of the rod soon replaced the wedge/slot mechanism because it gave a more secure engagement. These were more expensive, they still provided a relatively small contact area, and the problem of corrosion persisted. Furthermore, the bolts were subject to loosening, as the rock around the anchor weakened under stress. Thus, there are regulations requiring that this type of bolt be periodically retensioned.
The next advance in tensioned rockbolts was to back-fill the bore with grout after the rod was tensioned to achive a more secure engagement in the bore and to reduce corrosion. The rod was installed along with tubes through which grout could then be pumped. This bolt was expensive; special equipment was needed to make and handle the grout; correct installation required skilled labor; and the grout tubes were frequently broken during installation. Grout has also been used as the anchor for the rod, in which case the rod and grout tubes would be installed and grout pumped to surround the rod end. Once the grout set, the rod could be tensioned. The problems of handling the grout, of broken grout lines, and of the need for skilled labor persisted.
The latest advance in tensioned rockbolts has been the use of resin cartridges. Fast setting resin cartridges are inserted in the bore first, followed by slow setting resin cartridges. A rod is driven into the bore, bursting the cartridges. The faster setting resin at the top of the rod sets and anchors the rod so that it can be tensioned before the rest of the resin sets. With this system, a lot of expensive resin is required and special equipment is often needed to insert the cartridges and install the rod. The problem of pushing expensive resin into the dead space above the rod also persisted.
Applicants have developed a new type of rockbolt comprising a sleeve with ports at its upper end and containing cartridges of a hardenable binder, such as resin or grout, and a plunger slideably disposed in the sleeve. This rockbolt is simply inserted into the bore and held in place while the plunger is operated to extrude the binder from the ports in the upper end of the sleeve. The plunger can be operated, for example, by the application of air or water pressure, sources of which are readily available from construction or mining equipment likely to be on hand. Such sources may need to be augmented with a pressure intensifying apparatus, and such an apparatus can be mounted to a piece of other equipment or made portable. It is also possible to operate the plunger mechanically, such as by pushing it with a rod. Upon extrusion from the sleeve, the binder travels downward, enveloping the sleeve, and hardens, fixing the rockbolt.
This new rockbolt can also be used as a tensioned rockbolt. A mechanical anchor or expansion shell can be fixed to the top of the sleeve. The anchor can be set to engage the top of the bolt to the rock, the bolt tensioned, and then the plunger can be operated to extrude the binder and secure the sleeve. Another way to use this new rockbolt as a tensioned rockbolt is to put cartridges of fast setting binder in the bottom of the sleeve, so that the fast setting binder is the last to be extruded and remains near the top of the sleeve. Once the fast setting binder sets, the sleeve can be tensioned before the rest of the binder sets.
This invention combines the most desirable features of the prior rockbolts providing a tensioned rockbolt engaged in the surrounding rocks substantially along its length and encased in a binder. The entire rockbolt itself is self-contained. No grout or resin must be pumped into the bore. Thus, no additional parts or supplies are required, eliminating inventory and storage problems. Further, no special tools or equipment are needed to handle the grout or install the rockbolt, resulting in further savings. Because no special equipment is needed for installation, the length of the rockbolt is limited only by the height of the roof, in contrast to some prior rockbolts where installation equipment was placed directly below the bore, reducing the size of the bolt that could be inserted.
Because of the self-contained structure and simple operation, the installer does not need special skill or training. The installation is extremely fast and can be accomplished by just one worker. The quality of installation will not vary from bolt to bolt or depend upon the installer. Because of the simple structure, the rockbolt is relatively easy and inexpensive to manufacture. The same basic structure is easily adapted to a variety of rockbolts--both tensioned and untensioned.
The extrusion of a resin or grout binder results in a tensioned or untensioned rockbolt bonded along its length to the bore. The binder provides secure engagement of the rockbolt to the surrounding rock, but unlike the prior expanding rockbolts, the instant rockbolt is secured without radial stressing and cracking of the surrounding rock. The binder prevents corrosion of the rockbolt. The binder also prevents tensioned rockbolts from loosening, eliminating the need for periodic retensioning. Finally, the binder provides means for verifying the proper operation of the rockbolt, since when binder appears at the faceplate it indicates that the rockbolt has operated properly.
With this new design, the depth of the bore is not as critical as it was to prior grouted or resin rockbolts. The bore can be overdrilled and there will be no problem of pushing the grout or resin into the dead space above the rockbolt. Nor is the bore size or bolt size critical to proper mixing of the binder components as it was with the prior resin rockbolts. The resin is mixed as it is extruded, so the annulus size is not important to mixing. However, since resin or grout is only needed between the outside of the sleeve and the bore wall, by properly selecting the sleeve size, the amount and expense of binder can be significantly reduced. Because of this reduction in the amount of binder used, more effective epoxy type resin, previously too costly to be used, can be used. Thus, even better engagement between the rockbolt can be achieved than with the prior resined or grouted bolts.